1. Field of the Invention
This invention relates to an improved exposure control unit which is used advantageously in the exposing of light-sensitive materials such as photographic film or paper, either monochrome or color.
The improved control unit of the present invention may be used in the production of all types of photographs and particularly special purpose photographs, such as for advertising or other commercial or governmental purposes, where the photographs to be produced are not of the fixed magnifications ordinarily used in the exposing of photographic prints for the public.
2. Description of prior art
The object of the present invention is to provide an improved design for the apparatus claimed within U.S. Pat. No. 3,512,952. Wherein said apparatus presents electrical phenomenon that complicate construction and requires rare and expensive components that deem the design impractical for mass production, specifically, there are high quality precision capacitors with non-standard values employed in the capacitor network used for density factor. These capacitors must be made to specification by a manufacturer at an exorbitant cost. The apparatus claimed utilizes a phototube to discharge the capacitor network and a triode to monitor the voltage on the capacitor network, this arrangement has inherent leakage due to the high input impedance at the grid of the triode requiring careful construction techniques and low leakage components such as TEFLON.RTM. tube sockets and wire. There are no provisions on the claimed apparatus for range selection, it is customary to mask the phototube with light-attenuating neutral density filters in order to provide longer exposure times. If the operator requires shorter exposure times circuit modifications are required. Today a wide variety of photographic materials are available with many rated speeds making this a serious consideration for improvement.
The new design of the present invention is functionally identical from the operators viewpoint but constitutes an entirely different design approach.
The new design employs an integrator with an adjustable input resistance adjusted accordingly to the desired density. The integrator utilizes precision resistors and one high quality capacitor with a moderate tolerance that are readily available at a reasonable cost. The new design of the present invention eliminates high impedance nodes with a photodiode/operational amplifier front end connected to the input resistance of the integrator. This arrangement permits voltage control of the charging rate of the integrators capacitor, the voltage being a function of light intensity thus creating the integral of intensity relevant to both inventions. Part of the operational amplifier front end contains means for adjusting the proportionality of the voltage providing ranges for the exposure control system. Further advantages of the present invention are greater accuracy, reliability and stability attributed to its modern solid state construction.